This invention relates to methods and apparatus for detecting electrical defects in a semiconductor device or test structure. Additionally, it relates to generating an image of each identified defect for use in various defects analysis procedures, such as automatic defect classification.
Several different types of systems and techniques exist for identifying electrical defects, such as shorts and opens, on a semiconductor device or test structure. Commonly, test structures include probe pads that can be electrically probed to detect the existence of opens or shorts within such test structure. Another type of test structure is referred to as a voltage contrast type test structure. Electrical defects can be detected in a voltage contrast type test structure using an electron beam (e-beam) based tool. The e-beam based tool scans an electron beam over the test structure which causes portions of the scanned test structure to retain charge. Secondary and/or backscattered electrons are emitted from portions of the scanned test structure in response to the incident e-beam. The voltage contrast test structure is typically designed to have a first set of structures coupled to the substrate and a second set of structures which are left floating.
Several voltage contrast type test structures and methodologies for rapidly inspecting these test structures are described in U.S. Pat. No. 6,509,197, issued 21 Jan. 2003, by Akella V. S. Satya et al., which patent is incorporated herein by reference in its entirety. FIG. 1 illustrates a typical voltage contrast test structure 1100. This test structure 1100 includes alternating floating conductive lines 1104 and grounded conductive lines 1102 coupled to the substrate (not shown). One end (1105) of the conductive lines 1102 is coupled with the substrate, while the other end of both conductive lines 1102 and 1104 project into an initial scan area 1101. As the e-beam scans over the conductive line ends, the grounded and floating structures will obtain different voltage potentials in response to the e-beam. The potential differences in the grounded and floating conductive structures result in different secondary/backscattered electron intensities during the e-beam inspection. The different intensities of secondary/backscattered electrons emitted from the conductive structure are typically converted into a voltage contrast image such that a low potential portion of, for example, a conductive structure might be displayed as bright (intensity of the secondary electron emission is high) and a high potential portion might be displayed as dark (lower intensity secondary electron emission). Alternatively, the system may be configured such that a low potential portion might be displayed as dark and a high potential portion might be displayed as bright.
The voltage contrast type test structure is typically designed to produce a particular “expected” pattern of voltage potentials and brightness levels in an image generated during a voltage contrast inspection. Hence, when the image differs significantly from the expected intensity pattern, the test structure is identified as having a defect. In the example of FIG. 1, the conductive line ends which extend into the initial scan area 1101 are expected to appear as alternating bright and dark line ends. Opens and shorts within the conductive lines 1102 may be detected by scanning across the conductive line ends within the initial scan area 1101 in direction 1103. As shown, a conductive line 1104c which is designed to remain floating is shorted to an adjacent grounded structure 1102 so that conductive line 1104c appears bright, instead of having its expected dark appearance. After a defect is found within a particular conductive line, the defect may then be located more precisely by rotating the test structure relative to the e-beam and scanning the defective conductive line 1104c along direction 1106. The time required to rotate and realign the test structure can be significant.
Typically, a rapid e-beam scanning system, such as an eS20XP inspection tool available from KLA-Tencor, Corp of San Jose, Calif., is used to localize defects within a voltage contrast structure. Because conventional e-beam scanning tools cannot typically generate high resolution images of the defect, the voltage contrast test structure is transferred to a high resolution e-beam step and repeat system after a defect is localized. The e-beam step and repeat system is designed to generate a high resolution image of the defect that was previously localized with the e-beam scanning system. The resulting high resolution image of the defect may then be analyzed, e.g., classified. In an alternative methodology, defective structures are identified by probing the test structure with an electrical probe. The defect location within the failing test structure is typically identified in this methodology by using a scanning e-beam system. Subsequently, a high resolution image of the defect can be obtained using a step and repeat e-beam system. The localization of the defect within the failing structure can also be accomplished using a step and repeat system, however, at a significant time and throughput penalty.
The above described defect inspection techniques require multiple tools to localize and image the defects for classification. These methodologies also typically require rotation of the test structures in a scanning e-beam system thereby increasing the complexity of operation and negatively impacting the throughput. This produces systems that are complex and expensive. These techniques require multiple tools to perform the task because e-beam step and repeat systems are inefficient at localizing defects on a wafer and e-beam scanning and probe type systems provide poor or no high resolution images of the defects.
Accordingly, there is a need for improved methods for localizing electrical defects and if required generating a relatively high resolution image of electrical defects in semiconductor devices, test structures, and the like. Additionally, there is a need for improved voltage contrast test structures for facilitating such improved methods.